It is often desirable, for aesthetic and other reasons, to provide articles, such as architectural windows, automotive windows, eyewear lenses, and consumer electronics display pieces, among other items, that exhibit a desired color while still being transparent. Such color is sometimes obtained by the use of dyes that are included in the composition used to manufacture the glass or transparent plastic material. Alternatively, a colored film or coating containing a dye may be deposited upon the material. Pigments have been avoided because they typically produce hazy or opaque materials with reduced transparency.
Dyes, while often suitable for use in preparing colored transparent articles, also suffer from some drawbacks. Dyes are subject to degradation by visible light and/or ultraviolet light. In addition, dyes tend to migrate to the coating surface over time, thereby limiting the amount of dye that can effectively be included in the coating. As a result, materials colored with dyes are typically less durable and exhibit less colorfastness than comparable materials colored with pigments. Also, it is difficult to achieve strong colors using dyes, particularly at low film thicknesses.
The haziness of coatings produced from pigments often results from the use of pigment particles that have a size, often 0.5 micrometers (“microns”) or larger, that absorb light at certain wavelengths and scatter light at other wavelengths. This partial absorption and partial scattering creates a degraded coloration effect when viewed by an observer. Coatings that employ large amounts of such pigments, therefore, are not typically suitable for the production of a colored coating having low haze.
It is sometimes desirable, for cost and optical quality, among other reasons, to employ ultra thin coatings, i.e., coatings having a thickness of less than 2 microns. It would also sometimes be desirable for such coatings to exhibit both a strong color, i.e., a relatively high optical density at a given film thickness, while maintaining low haze, i.e., a relatively high level of transparency. Attainment of coatings with strong color, however, typically requires the use of a relatively large amount of colorant, such as pigment, which, as indicated above, is problematic when using pigment particles that exhibit degraded coloration effects.
The present invention relates to the surprising discovery that coating compositions containing certain organic pigments can be used in relatively large quantities, i.e., 20 percent by weight or more, based on the total weight of the composition, to produce ultra thin coatings that exhibit low haze and a strong color.